Plastic semiconductor packages are fabricated using lead frames. A conventional plastic package includes a semiconductor die attached to a lead frame, and encapsulated in a plastic body. The lead frame supports the die during a molding process, and provides the internal traces and terminal leads of the completed package. Typically, the lead frame comprises a stamped or etched metal, such as a nickel-iron alloy, or a copper based alloy. Lead frames are provided in strips adapted to form multiple packages, which are subsequently singulated into individual packages.
One type of lead frame includes mounting paddles for the dice, and patterns of lead fingers spaced around the peripheries of the mounting paddles. Prior to the molding process, the backsides of the dice can be attached to the mounting paddles using an adhesive. In addition, metal wires can be wire bonded to the lead fingers, and to bond pads on the faces of the dice. Using a transfer molding process, a plastic material, such as an epoxy resin, is molded to either side of the dice and lead frame, and over the wire bonded wires. The lead frame is then singulated into multiple packages, and the lead fingers are trimmed and shaped to form the terminal leads of the packages.
One approach to fabricating a plastic semiconductor package involves providing the lead frame with a mounting paddle that is recessed from a center line of the package. This type of semiconductor package 10 is illustrated in FIGS. 1A-1C. In FIG. 1A, the package 10 includes a semiconductor die 12 and a lead frame 14. A backside of the die 12 is attached to a mounting paddle 18 of the lead frame 14. In addition, wires 16 are bonded to bond pads on the die 12, and to lead fingers of the lead frame 14. For simplicity the lead fingers are not shown in FIGS. 1A-1C. However, the lead fingers are spaced around a periphery of the mounting paddle 18, in a plane coincident to a major plane of the lead frame 14. The package 10 also includes a plastic body 20 molded to either side of the die 12 and lead frame 14.
To minimize thermal stresses resulting from a TCE mismatch between the plastic body 20, and the lead frame 14, it is preferable to locate the lead frame 14 along a center line 22, or neutral bend axis, of the package 10. The center line 22 is also coincident to a major plane of the lead frame 14. To improve the wire bonding process, it is also desirable to locate the die 12 below this center line 14. This reduces drag forces on the wires 16 during wire bonding, and allows the wires to be more easily encapsulated by the plastic body. In order to satisfy these requirements, and properly locate the die 12 with respect to the lead frame 14, the mounting paddle 18 can be offset from the center line 22 of the package 10.
A downset 24 can be provided on the lead frame 14 to recess the mounting paddle 18 below the center line 22 of the package 10. As shown in FIG. 1B, the downset 24 has a length "L" and a depth "D". The depth "D" is equal to an offset of the die 12 below the center line 22 of the package 10. As used herein the term "downset" refers to a stepped portion of a lead frame, that provides an offset for a first portion of the lead frame relative to a second portion of the lead frame. Typically, the downset provides an offset for a mounting paddle, or other die mounting portion of the lead frame relative to lead fingers on the lead frame.
Some recent package designs require a relatively large offset for the die 12 and thus relatively large downsets 24. For example, a depth "D" of the downsets 24 can be from 7 mils to 14 mils. One problem with this construction is that the planarity of the mounting paddles 18 can be effected. As shown in FIG. 1C, the mounting paddle 18 is in effect suspended from the downsets 24. Any differences in the depth "D" of the downsets 24, or in the parallelism or locations of the downsets 24, can cause the mounting paddle 18 to be angled with respect to the lead frame 14.
Another problem with large downsets 24 is that the lead frames 14 are often stacked for handling. The large downsets 24 can cause the lead frames 14 to interlock and become difficult to separate. Also, lead frames 14 that are fabricated by stamping can include downsets 24 which require overworking and work hardening of the metal which forms the downsets. This can cause cracks and weak spots in the downsets 24.
In view of the foregoing deficiencies in lead frames, the present invention is directed to an improved downset lead frame and to an improved method for fabricating semiconductor packages using the downset lead frame.